The invention relates to a method of optimizing the charging of a battery of storage cells, and to apparatus for performing the method. Batteries of storage cells are used to supply numerous items of equipment with electrical energy when the public electricity mains fails. For example: automated equipment, data-processing hardware, telecommunications exchanges, etc. are backed up for a limited duration, by means of batteries of storage cells.
"Charging" consists in storing electrical energy in the battery in electrochemical form. The length of time during which the battery fulfills its backup function is limited in particular by the quantity of energy stored in the battery in electrochemical form, with "discharging" the battery consisting in giving back, in the form of electrical energy, the energy stored in electrochemical form. Optimizing the charging of a battery therefore consists in storing as much electrical energy as possible in electrochemical form. Furthermore, it is unnecessary and detrimental to supply a battery with electrical energy which cannot be converted into stored electrochemical energy. When a battery has stored the maximum possible energy in electrochemical form, "overcharging" phenomena appear, consisting in particular in electrolysis and a rise in temperature, or even thermal runaway.
Outside the charging periods, or the periods of discharging to an external circuit, a battery undergoes internal self-discharging which slowly dissipates the stored energy. In order to maintain its charged state, the battery is subjected to a "maintenance" mode making up for self-discharging by supplying the battery with a predetermined low current.
In conventional charging methods, the change-over from maintenance mode to charging mode takes place once only after each discharging period, and the change-over is selected as a function of certain parameters considered separately or in combination: the quantity of electricity supplied during the discharging period, or the duration of the discharging period. The change-over from charging mode to maintenance mode is also selected once only, as a function of certain parameters considered separately or in combination: the quantity of electricity supplied to the battery during the charging period; the charging current; the back-electromotive force of the battery; the temperature of the battery; the temperature difference of the battery above ambient temperature; and the duration of charging.
Conventional charging methods consist in going from charging mode to maintenance mode when overcharging actually takes place and can then be detected by means of the temperature of the battery or by means of the difference between the temperature of the battery and ambient temperature.
Other conventional charging methods consist in assuming that the charging capacity, i.e. the maximum amount of energy which can be stored in electrochemical form, is always equal to the reference capacity of the battery, which reference capacity is by definition the quantity of electrical energy that can be given back by the battery in the event that the battery has been optimally charged. The decision to switch from charging mode to maintenance mode is then taken either when the duration of charging at constant current reaches a fixed value; or when the electricity balance, i.e. the net quantity of electricity supplied by the battery to an external circuit, reaches a fixed value. In certain cases, the maximum storable energy is much less than the reference capacity because of a rise in ambient temperature, or because of battery ageing. After this maximum has been reached, the battery is subjected to overcharging.
These methods therefore suffer from the drawback of supplying energy unnecessarily to the battery during a certain length of time, at least in certain cases, and the energy may be detrimental to the length of life of the battery. All known charging methods further suffer from the drawback of not providing optimum charging in certain cases, because the charging capacity of the battery may vary as a function of various parameters, in particular temperature, whereas the discharging capacity remains equal to the reference capacity, because it depends almost entirely on the quantity of energy actually stored in electrochemical form, for the usual fields of use and for determined discharging conditions, in particular the discharging current.
The variation in the charging capacity as a function of the temperature of the battery depends on the manufacturing technology of the battery. Generally, charging capacity decreases when temperature increases. Consequently, a battery charged using a conventional method may reach its overcharging limit if charging conditions are unfavorable, i.e. if ambient temperature is high, whereas more favorable charging conditions may subsequently occur, in particular if the ambient temperature decreases. The capacity actually charged remains the capacity obtained during charging under unfavorable conditions. The capacity actually charged is therefore less than the capacity that the battery could obtain under new conditions that are more favorable.